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Full text of "The Cambridge Natural History"

CLAVICLE

ently not due to real resemblance. What has happened in the
Monotremata is, that the prescapular fossa is so enormously
expanded that it occupies the whole of the inner side of the
blade-bone, while the subscapulur fossa which, so to speak, should
occupy that situation, has been thus pushed round to the front,
where it is divided from the postscapular fossa by a slight
ridge only.

The clavicle is a bone which varies much in mammals. It is
sometimes indeed, as in the Ungulata, entirely absent; in other
forms it shows varying degrees of retrocession in importance ; it is
only in climbing, burrowing, digging, and flying mammals that
ifc is really well developed

In the higher Mammalia the coracoid* is present, but does
not reach the sternum as in the Monotremata. It is known to

human anatomists as
the coraeoid process
of the scapula. It
has been found, how-
ever, by Professor
Howes2 and others,
that this process
really consists of
two separate centres
of ossification, form-
ing two separate
bonelets, which in.
the adult become
firmly ankyloscd to
each other and to
the scapula. These

Fia. 29.—Shoulder girdle, with upper end of sternum (inner
surface) of Shrew (Sorex), alter Parker, x 7. a, Acro-
mion ; c, coracoid ; clt clavicle j ec, partially osHilied
" epicoracoid" of Parker, or rudiment of the sternal
extremity of the coracoid ; ma, metacronrial process ;
tns8t ossified " mesoscapular segment"; ast, omo&ternum ;
pc, rudiment of precoracoid (Parker) ; ps, presternum ;
sr\ first sternal rib; sr2, second sternal rib. (From
Flower's Osteology.)
two separate bones have been met with in the embryo of .
Sciurus, and the young of various other mammals belonging to very
diverse orders, such as Edentates and Primates. The separation even
occasionally persists in the adult. The question is, What is the
relation of these bonelets to the coracoid of the Monotremata and
to the corresponding regions of reptiles ? Professor Howes terms the
IQWW patch of bone the metacoracoid and the upper the epieoraeoid;
1 To this category are perhaps to be referred cartilaginous pieces occurring in
tlie Rabbit, Mus and JSorese (see Fig. 29 above).
a " On the Coraeoid of the Terrestrial Vertebrates," P.Z.8* 1893, p. 585,